Optical fiber and planar waveguide technology are becoming the transmission mediums of choice for many communication networks because of the speed and bandwidth advantages associated with optical transmission. In addition, wavelength division multiplexing (WDM) is being used to meet the increasing demands for higher data rates and more bandwidth in optical transmission applications.
In its simplest form, WDM is a technique whereby parallel data streams of modulated light of different wavelengths in the form of channels are coupled simultaneously into the same optical fiber. As such, a WDM signal is typically viewed, as a composite optical signal comprised of a plurality of optical wavelength channels sharing a single transmission medium, each wavelength channel having a different center wavelength of light. Although each wavelength channel actually includes a range of wavelengths making up the channel width, it is common to refer to an optical wavelength channel in terms of its center wavelength.
It is often necessary to add or remove a particular wavelength channel at various points along an optical fiber transmission path, without significantly disturbing or disrupting the remaining wavelength channels, that is, in a substantially “hitless” manner, whether the optical transmission system is a long haul, metropolitan, or local. Adding or removing a wavelength channel is accomplished utilizing add/drop devices. An add/drop device typically utilizes a bandpass filter, that is, an optical filter that is transmissive with respect to one or more wavelength channels and reflective with respect to the remaining wavelength channels, to add or remove the desired wavelength channel. The remainder of the wavelength channels not within the passband of the filter, remain unaffected by the device, and the transmission of their respective modulated light data streams is unimpeded.
In recent years, tunable filters have been developed which, when incorporated as the optical filter in an add/drop device, enable the device to be dynamically tuned to add or remove a desired optical wavelength channel from the plurality of wavelength channels. In the instance where it is desired to change the added or dropped wavelength channel, it is easily accomplished without having to replace the filter element, or the entire add/drop device with another having the desired bandpass characteristics. This is typically accomplished by repositioning the filter with respect to an incident optical beam. However, one unfortunate aspect resulting from dynamically tuning an add/drop device is that intermediate wavelength channels, those channels having wavelengths existing between that of the initially tuned channel and that of the finally tuned channel, will each sequentially exhibit a temporary loss of signal continuity as the filter is tuned to each respective channel's wavelength. This occurs because the point of incidence of the optical beam upon the filter, in transitioning from a point corresponding to the initially tuned channel to a point corresponding to the finally tuned channel, passes filter locations corresponding to each of the intermediate channels. When the composite optical signal light strikes filter locations corresponding to intermediate wavelength channels, intermittent data loss from those intermediate wavelength channels results. Such an intermittent loss of data is often referred to as a so-called “hit.” The deleterious effects of a data hit, to the devices for which the data is intended to be transmitted, are well known. Devices exposed to such a data loss must either compensate for the loss of data, or request retransmission of the lost data. Ultimately, such data loss results in diminished quality of service, decreased bandwidth efficiency, or both.
Although known so called “hitless filters” may not be entirely hitless, data loss is reduced and bandwidth efficiency improved while tuning an optical add/drop device having a substantially hitless wavelength-tunable optical filter. For example, U.S. Pat. No. 6,292,299, filed in Feb. 14, 2000 and issued Sep. 18, 2001 in the name of Liou; Kang-Yih, assigned to Lucent Technologies Inc. incorporated herein by reference, describes a hitless wavelength-tunable optical filter that includes a broadband reflective region and a tunable filter region. The so called hitless tuning of the device is accomplished by changing the point at which an optical beam is incident upon the filter region along a constant wavelength channel track whenever the beam strikes the device in the filter region. Realignment to a position associated with a new wavelength channel track is performed when the optical beam is incident upon the broadband reflective region. Repositioning the optical beam to the filter region occurs at a location corresponding to the new wavelength to be added/dropped and subsequent optical beam realignment within the filter region is along the new wavelength channel track. Although the hitless wavelength-tunable optical filter disclosed by Liou appears to perform its intended function, there are believed to be limitations to this filter for which this invention provides solutions.
Another US patent to an invention essentially the same as that of Liou, is disclosed in U.S. Pat. No. 6,320,996 with a provisional priority date of Dec. 31, 1998, and is now assigned to JDS Uniphase.
Ideally, a hitless filter should have negligible or “no” loss incurred in the express channels when a single channel is being dropped or added. Notwithstanding, data transmission errors may result in the example using a mirror 120 as shown in FIG. 1 of U.S. Pat. No. 6,292,299. The potential problem occurs when the beam and/or filter are moved relatively wherein the beam makes a transition from the un-mirrored portion 110 to the mirrored portion 120 such that different portions of the same beam impinge upon the two portions 110 and 120. This will occur when the optical beam moves between points 1 and 2, or 3 and 4 due to a phase discontinuity between the mirrored and un-mirrored portions.
One skilled in the art could suggest placing the mirror at the opposite side of the filter with respect to the incident optical beam signal. This would eliminate the phase discontinuity described in the previous paragraph, for all express channels, in this instance, those reflected.
In that case however, the central wavelength corresponding to any given position of the optical beam on the mirrored area will suffer temporal delay when compared to all other channels. This delay will correspond to the light traveling back and forth across the whole filter for that central wavelength channel and, this would result in coupling loss for that channel. The coupling loss, or “hit”, will travel from channel to channel as the beam is translated from point 2 to point 3 in FIG. 1.
It is an object of this invention to lessen the effect of the phase discontinuity for the optical beam signal crossing between the mirrored and un-mirrored portions, while lessening optical path length delays for the optical beam signal incident over the mirrored portion.
It is an object of this invention to provide a variable filter that is substantially “hitless” lessening both an unwanted phase delay and an optical path length delay simultaneously.